The present invention relates to the control of the operation of hydraulic cylinder-and-piston units in general, and more particularly to an arrangement for supervising and controlling synchronous displacement of the pistons of a hydraulic advancement unit and a hydraulic roof cap displacement unit employed in a mine roof support shield arrangement used in underground mining.
There are already known various constructions of control arrangements for controlling the operation of the hydraulic cylinder-and-piston units employed in the above-mentioned mine roof support shield arrangement. In such known arrangements, each of the hydraulic units includes a cylinder bounding a piston space next to one axial face of the respective piston and an annular space surrounding a piston rod which extends from the other end face of the respective piston to the exterior of the respective unit, and pressurized hydraulic medium is selectively supplied, in dependence on the desired direction of the displacement of the respective piston, either into the piston space or into the annular space of the respective unit to provide a pressure differential which is needed for displacing the piston in the desired direction, while the spent hydrualic fluid is discharged from the respective other space. It is also known to use a switching valve for interchanging the connections between the pressurized medium supply conduit and the return conduit, on the one hand, and the conduits which lead to the respective spaces of the hydraulic units, on the other hand, to thereby change the directions of displacements of the respective pistons.
During the movement of, for instance, a mine face conveyor, which also serves as a counterbearing for a following displacement of the protective shield arrangement, and especially during the advancement of such conveyor toward the mine face which recedes as the material being mined is removed from such mine face, the protective roof support shield arrangements remain, as a rule, in a bracing engagement with the mine floor and the mine roof for a certain period of time. During the mining operation, the distance between the main roof cap of the shield arrangement and the mine face gradually increases. This distance is bridged by respective displaceable roof caps. In this connection, it is desirable for the respective displaceable roof cap to be gradually displaced or extended beyond the main roof cap toward the mine face to the same extent as the mine face conveyor is advanced relative to the protective roof support shield arrangements.
For this purpose, there are already known purely hydraulic synchronous run systems which make sure that the displacement cylinder-and-piston units which displace the displaceable roof caps are always actuated in such a manner that the displaceable roof caps are extended to an extent corresponding to the displacement of the piston of the respective advancement cylinder-and-piston unit which is interposed between the protective roof shield arrangement and the mine face conveyor. However, purely hydraulic synchronous run systems are, of necessity, rather bulky so that they require a considerable volume of available space for their accommodation.
In order to reduce the demand for the accommodation space for the control arrangement, there have already been proposed combined electrical and hydraulic synchronous run control systems. In synchronous run control systems of this type, respective electrical or electronic displacement measurement devices are associated with the advancement cylinder-and-piston units as well as with the displacement cylinder-and-piston units, such devices being then connected with a programmable electronic control and regulating arrangement. The annular and piston spaces of the advancement and displacement cylinder-and-piston units are then connected via respective multi-way valves to the hydraulic supply and return conduits. While it is true that it was possible, by using the electronic control and regulating arrangement, to reduce the overall space consumption for the control arrangement, the multi-way valves which are being used in this connection are still quite expensive both as to their sizes and their number. This, in turn, necessitates the provision of a correspondingly high number of connection and interconnection conduits.